Alumina agglomerates, the preparation method thereof and use of same as an absorbent or catalyst carrier

ABSTRACT

The invention relates to alumina agglomerates of the type obtained by dehydrating an aluminium oxyhydroxide or hydroxide, agglomerating the alumina thus obtained, hydrothermally treating the agglomerates and calcinating same. Said agglomerates are characterised in that: the V 37 Å  thereof is greater than or equal to 75 ml/100 g, preferably greater than or equal to 80 ml/100 g and, better still, greater than or equal to 85 ml/100 g; the V 0.1 μm  thereof is less than or equal to 31 ml/100 g; and the V 0.2 μm  thereof is less than or equal to 20 ml/100 g, preferably less than or equal to 15 ml/100 g and, better still, less than or equal to 10 ml/100 g. The invention also relates to a catalyst carrier, an intrinsic catalyst or an absorbent, in particular for use in the petroleum and petrochemical industry, comprising such alumina agglomerates. Moreover, the invention relates to methods for preparing said agglomerates.

The invention relates to the field of alumina agglomerates that can beused especially as adsorbents, as catalyst supports or as catalysts.More precisely, it relates to novel alumina agglomerates having specificphysical characteristics, in particular from the standpoint of theirporosity, giving them particularly advantageous mechanical properties.The invention also relates to a process for obtaining such agglomerates.Finally, it relates to their use as catalyst supports, as catalysts oras adsorbents.

Certain catalytic processes, such as heterogeneous catalysis, forexample in the field of treatment of petroleum oil cuts or the treatmentof gaseous effluents, such as the exhaust gases from internal combustionengines, require the use of supports having a high porosity and goodmechanical properties, such as good crush resistance and/or attritionresistance.

Alumina-based supports meet these criteria, especially when they haveundergone a hydrothermal treatment, for example after the alumina hasbeen made in the form of agglomerates. This is because such a treatmentsubstantially improves the mechanical properties of the agglomerates.

This hydrothermal treatment consists in impregnating the aluminaagglomerates with water or an aqueous acid solution, and then heatingthese agglomerates, placed inside an autoclave, to a temperature ofabove 80° C. European Patent No. EP-A-0 387 109, in the name of theApplicant, describes an example of an advantageous process for carryingout this hydrothermal treatment.

The agglomerates thus treated are then dried and then subjected to aheat treatment called “calcination” at a given temperature. Thiscalcination temperature is chosen according to the desired specificsurface area and to the temperature at which these supports will beused.

The efficiency of such alumina agglomerates is to a large part dependenton their porosity: a high porosity is often favorable for the variousapplications envisioned, because of the lower diffusional and/ortortuosity constraints. In applications in which the support will beused not only for satisfactorily dispersing a metallic active phase orphases supplied during production of the catalyst, but also to retaincontaminants, such as during a hydrometallation operation, a highporosity is advantageous. However, a high porosity is generally regardedas going hand in hand with a degradation of the mechanical strength ofthe material. The use of alumina agglomerates with a high porosity istherefore difficult in important applications, specially in catalystsupports for which the mechanical strength of the alumina to a largeextent governs the duration of use of the catalyst. It turns out thatdegradation of the mechanical strength of the catalyst during use hasdramatic effects, both technically and economically, on the operatingprocess.

The object of the invention is to provide alumina agglomerates userswith materials, exhibiting an excellent compromise between high porosityand mechanical strength that makes them compatible with demandingapplications from the latter standpoint.

For this purpose, the subject of the invention is alumina agglomeratesof the type obtained by the treatment of an aluminum hydroxide oroxyhydroxide, agglomeration of the alumina thus obtained, hydrothermaltreatment of the agglomerates and calcination, characterized in that:

-   -   they have a V_(37 Å) of greater than or equal to 75 ml/100 g,        preferably greater than or equal to 80 ml/100 g and even more        preferably greater than or equal to 85 ml/100 g;    -   they have a V_(0.1 μm) of less than or equal to 31 ml/100 g,        preferably less than or equal to 25 ml/100 g, even more        preferably less than or equal to 20 ml/100 g and optimally less        than or equal to 15 ml/100 g; and in that:    -   they have a V_(0.2 μm) of less than or equal to 20 ml/100 g,        preferably less than or equal to 15 ml/100 g and even more        preferably less than or equal to 10 ml/100 g.

Preferably, they have a V_(1 μm) of less than or equal to 7 ml/100 g,preferably less than or equal to 5.5 ml/100 g and even more preferablyless than or equal to 4 ml/100 g.

Preferably, they have a V_(0.1 μm)/V_(0.2 μm) ratio of greater than orequal to 1.5, preferably greater than or equal to 2 and even morepreferably greater than or equal to 2.5.

Preferably, they have simultaneously a V_(37 Å) of greater than or equalto 80 ml/100 g, a V_(0.1 μm) of less than or equal to 15 ml/100 g, aV_(0.2 μm) of less than or equal to 10 ml/100 g, a V_(1 μm) of less thanor equal to 4 ml/100 g and a V_(0.1 μm)/V_(0.2 μm) ratio of greater thanor equal to 2.5.

Preferably, they are obtained from dehydrated hydrargillite.

Preferably, they are in the form of beads or of extruded materials, orof crushed materials, or in the form of monoliths.

The subjects of the invention are also a catalyst support, especiallyfor the petroleum or petrochemical industry, consisting of aluminaagglomerates of the abovementioned type, an intrinsic catalyst,especially for the petroleum or petrochemical industry, consisting ofalumina agglomerates of the abovementioned type, and an adsorbent,especially for the petroleum or petrochemical industry, consisting ofalumina agglomerates of the abovementioned type.

The subject of the invention is also a process for producing aluminaagglomerates of the abovementioned type, in which:

-   -   an aluminum hydroxide or oxyhydroxide, preferably hydrargillite,        undergoes flash dehydration in order to obtain an active alumina        powder;    -   said active alumina powder undergoes a forming operation so as        to obtain beads with a green fill density of between 500 and        1100 kg/m³, preferably between 700 and 950 kg/m³ inclusive, and        a diameter predominantly between 0.8 and 10 mm, preferably        between 1 and 5 mm;    -   said beads undergo a heat treatment so as to provide them with a        specific surface area of between 50 and 420 m²/g;    -   said beads undergo a hydrothermal treatment by impregnation with        water or an aqueous solution, preferably aqueous acid solution,        followed by residence in an autoclave at a temperature of above        80° C.; and    -   the agglomerates thus obtained are calcined, preferably between        500 and 1300° C.

After the heat treatment that follows granulation, the alumina beadspreferably have a V_(37 Å) of greater than 65 ml/100 g, preferablygreater than 70 ml/100 g.

The active alumina powder obtained after dehydrating the aluminumhydroxide or oxyhydroxide is preferably ground in order to obtain apowder with a median particle size d₅₀ of preferably between 5 and 25μm.

The active alumina powder obtained after dehydrating the aluminumhydroxide or oxyhydroxide is washed, preferably with water or an aqueousacid solution.

Before granulation of said active alumina powder, the latter preferablyundergoes a flash operation.

The subject of the invention is also a process for producing aluminaagglomerates of the abovementioned type, in which:

-   -   an alumina-based material is mixed and extruded in order to form        it;    -   the extruded materials thus obtained are subjected to a heat        treatment so as to give them a specific surface area of between        50 and 420 m²/g;    -   said extruded materials are subjected to a hydrothermal        treatment by impregnation with water or with an aqueous        solution, preferably an aqueous acid solution, followed by        residence in an autoclave at a temperature of above 80° C.; and    -   the agglomerates thus obtained are calcined, preferably between        500 and 1300° C.

Said alumina-based material is preferably dehydrated hydrargillite.

Said alumina-based material may also come from the precipitation ofboehmite, pseudo-boehmite or bayerite, or a mixture of such materials.

During the forming of said alumina powder or of said alumina-basedmaterial in one of the above processes, one or more pore-formingmaterials that disappear on heating are preferably added to it.

Said pore-forming materials are preferably chosen from wood flour,charcoal, sulfur, tars, plastics or emulsions of plastics, polyvinylalcohols, naphthalene.

Said hydrothermal treatment is preferably carried out at a temperatureof 150 to 270° C., preferably from 170 to 250° C., for a time of greaterthan 45 minutes, preferably from 1 to 24 hours and even more preferablyfrom 1.5 to 12 hours.

Said hydrothermal treatment is preferably carried out using an aqueousacid solution containing one or more mineral and/or organic acids.

Said aqueous acid solution also preferably includes one or morecompounds that can release anions capable of combining with the aluminumions.

As will have been understood, the invention consists in obtaining, inthe case of alumina agglomerates, a particular distribution of thevolumes occupied by the pores of various diameter classes. It is underthese conditions that mechanical strength properties equal to, or evenbetter than, those of the usual agglomerates having an equivalent orlower overall porosity are obtained. This particular distribution may beobtained using an agglomerate manufacturing process that isdistinguished from the usual processes by the execution of the aluminaforming step according to the particular methods of implementation thatwill lead to agglomerates having the desired characteristics finallybeing obtained. This forming operation is followed by a shorthigh-temperature heat treatment, or even also preceded by such atreatment.

The invention will be more clearly understood on reading the descriptionthat follows, given with reference to the single appended FIGURE. Thelatter shows the pore distribution of an alumina A obtained asintermediate product in an example of implementation of the processaccording to the invention and the pore distribution of a final aluminaB according to the invention, obtained from this alumina A.

In the rest of the description, the various parameters that will bereferred to are defined below.

The morphology of the aluminas is defined by the indicator of thevolumes occupied by the pores having a diameter of greater than or equalto a series of given diameters, namely:

-   -   the volume occupied by the pores having a diameter greater than        or equal to 37 Å (V_(37 Å));    -   the volume occupied by the pores having a diameter of greater        than or equal to 0.1 μm (V_(0.1 μm));    -   the volume occupied by the pores having a diameter of greater        than or equal to 0.2 μm (V_(0.2 μm)); and    -   the volume occupied by the pores having a diameter of greater        than or equal to 1 μm (V_(1 μm)).

These volumes may be measured conventionally by the technique called“mercury porosimetry”.

For this purpose, the alumina specimen is placed in a column into whichmercury at a pressure P is introduced. Since mercury does not wet thealumina, its penetration or its non-penetration into the pores of thespecimen having a given diameter depends on the value of P. The finestpores require, in order to be filled, a higher pressure P to beestablished than that for filling the coarser pores. By measuring theamount of mercury penetrating the specimen for various values of P it ispossible to determine the volume occupied by the pores of a diametergreater than the given values of this diameter.

The mechanical strength of the specimen is measured by two parameters:the measured particle-to-particle crush strength (S_(pp)) and themeasured value in the shear test.

To measure the S_(pp) of agglomerates, said agglomerates are initiallypretreated for two hours at 300° C. in an oven and then left to cool ina dessicator. They are then taken one by one and placed between thehammer and the anvil of a compressometer until fracture. The finalresult is expressed in the form of a mean fracture force measured on apopulation of about twenty agglomerates.

The objective of the shear test is to determine the mechanicalresistance to friction of spherical agglomerates. The specimen is placedin a ring-shaped cell and pressure is applied to the alumina by means ofa cylinder having the same shape as the cell, which is then rotated. Thespecimen thus undergoes shear forces leading to a reduction in the sizeof the beads because of the rotation of the cell under the pressureexerted, the base of the cylinder and the bottom of the cell being alsonotched. The analysis reported here was carried out under a pressure of0.4 bar, the shear being 50. The results are expressed as the percentageundersize for a screen with 1.7 mm square meshes.

A process for manufacturing alumina agglomerates according to theinvention will now be described.

The process starts, as in previously known processes, with intense“flash” heating resulting in sudden dehydration of an aluminum hydroxide(hydrargillite, gibbsite or bayerite) or of an aluminum oxyhydroxide(boehmite or diaspore) by means of a stream of hot gas for removing andcarrying away the evaporated water very rapidly. The temperature isaround 400 to 1200° C. and the time during which the material to bedehydrated is in contact with the gases is from the order of a fractionof a second to four or five seconds. Active alumina in powder form,having a median particle size d₅₀ of about 40 μm, is thus obtained. Asstarting compound, it is preferred to use hydrargillite. Experimentshave shown that this compound is the most favorable for obtaining afinal product having the desired properties. In addition, it isrelatively inexpensive.

The powder obtained then undergoes, preferably and as is known, agrinding operation that brings its median particle size d₅₀ to about 15μm, generally between 5 and 25 μm.

Next, as is known, the powder is washed with water or with acidifiedwater, this having in particular the purpose of reducing its alkalimetal content.

According to the invention, the powder then preferably undergoes a flashoperation under conditions similar to those of the flash operationdescribed above so as to continue to develop its porosity.

Next, according to the invention, an operation of forming the alumina iscarried out. According to the preferred method of implementing theinvention, the alumina powder is granulated by means of a rotationaltechnique such as, for example, a rotary pelletizer or rotary drum. Thistype of process makes it possible to obtain beads of controlled porediameters and distribution, these dimensions and distributions generallybeing created during the agglomeration step. The porosity may be createdby various means, such as the choice of particle size of the aluminapowder or the agglomeration of several alumina powders of differentparticle sizes. Another method consists in mixing with the aluminapowder, before or during the agglomeration step, one or more compounds,called pore formers, that disappear when heated and thus create porosityin the beads. As pore-forming compounds used, mention may be made, byway of example, of wood flour, charcoal, sulfur, tars, plastics oremulsions of plastics, such as polyvinyl chloride and polyvinyl alcohol,naphthalene or the like. The amount of pore-forming compounds added isdetermined by the desired volume.

The aim is to produce beads with a green fill density of between 500 and1100 kg/m³, preferably between 700 and 950 kg/m³, for example 810 kg/m³,and a diameter of predominantly between 0.8 and 10 mm, preferablybetween 1 and 5 mm. This granulation is itself a conventional process,but the addition of the invention lies essentially in the choice of thecharacteristics of the beads resulting from the granulation. Thesecharacteristics, in combination with the subsequent treatments undergoneby the beads, will determine the properties of the agglomerates obtainedat the end of the treatment.

Next, again according to the invention, the beads undergo a heattreatment that allows an alumina to be obtained whose specific surfacearea is from 50 to 420 m²/g. Below 50 m²/g, the reactivity of thealumina during the subsequent autoclaving operation would not besufficient, and a specific surface area of greater than 420 m²/g wouldcorrespond to pores in the final product that are too small for theapplications envisioned. This alumina will be called hereafter “aluminaA”. An example of the pore distribution of such an alumina A is shown inthe single figure. In this example, the specific surface area of thisalumina A is 187 m²/g. In this single FIGURE, the pore diameter (in μm)is plotted on the x-axis and the cumulative pore volume (in ml/100 g),that is to say the volume occupied by the pores having diameters greaterthan or equal to the diameter on the x-axis, is plotted on the y-axis.

To obtain a final alumina according to the invention, it is recommendedto give alumina A a V_(37 Å) of greater than 65 ml/100 g, usuallygreater than 70 ml/100 g.

This alumina A then undergoes a hydrothermal treatment similar to thoseknown in the prior art, for example in the aforementioned documentEP-A-0 387 109. To give an example, this treatment may consist ofimpregnation of alumina A with an aqueous solution containing 4%aluminum nitrate and 9% formic acid (these percentages being calculatedon a weight basis with respect to the weight of alumina introduced),followed by residence of the impregnated alumina A in an autoclave witha rotating basket at 200° C. for 5 h 30 min. Generally speaking, thehydrothermal treatment is carried out at a temperature of preferablybetween 150 and 270° C., advantageously between 170 and 250° C.inclusive. The duration of said treatment is in general greater than 45minutes, preferably between 1 and 24 hours and even more advantageouslybetween 1.5 and 12 hours inclusive. The aqueous acid impregnationsolution comprises one or more mineral and/or organic acids. To give anexample, mention may be made of nitric acid, hydrochloric acid,perchloric acid, sulfuric acid and weak acids, the solution of which hasa pH of less than 4, such as acetic acid or formic acid. It may alsocontain one or more compounds that can release anions capable ofcombining with the aluminum ions. Thus, by way of example, mention maybe made of compounds comprising a nitrate (such as aluminum nitrate),chloride, sulfate, perchlorate, chloroacetate, trichloroacetate,bromo-acetate or dibromoacetate ion, and the anions of general formula:R—COO⁻ such as formates and acetates.

Finally, as is usual, the alumina undergoes a final heat treatment (orcalcination) at high temperature (typically between 400 and 1300° C.,for example at 800° C.), which allows the desired specific surface areato be obtained.

Beads are the preferred form of the invention, but it is alsoconceivable to use, for example, materials in extruded form or crushedmaterials or monoliths, and also to use alumina in powder form.

According to an alternative method of implementing the invention, thealumina may also be in the form of alumina extrudates. These aregenerally obtained by mixing and then extruding an alumina-basedmaterial followed by calcination and then, in succession, as describedabove, impregnation, hydrothermal treatment and then post-forming heattreatment. The starting material may vary very widely in nature: it mayresult from the partial and rapid dehydration of hydrargillite, or fromthe precipitation of boehmite, pseudo-boehmite, bayerite or of a mixtureof these aluminas. During mixing, the alumina may be mixed withadditives, especially pore formers such as those defined above.

The single FIGURE shows the pore distribution of an alumina according tothe invention, called “alumina B”, produced according to the particularexample of hydrothermal and calcination treatment that was described inthe case of the above alumina A. The calcination gave this alumina B aspecific surface area of 110 m²/g.

The single FIGURE shows that the hydrothermal and calcination treatmentoperations resulted in an alumina B being obtained in which, comparedwith the starting alumina A, the volume occupied by the pores of smallsize (<0.04 μm) has substantially increased. Above all, the volumeoccupied by the pores having a size greater than 0.2 μm decreasedconsiderably, to the point of becoming almost negligible.

The aluminas according to the invention are preferably obtained fromhydrargillite; they have a V_(37 Å) of at least 75 ml/100 g, preferablygreater than 80 ml/100 g and even more preferably greater than 85 ml/100g. At the same time, their V_(0.1 μm) is less than or equal to 30 ml/100g, preferably less than or equal to 25 ml/100 g and even more preferablyless than 20 ml/100 g, or even less than 15 ml/100 g. They have aV_(0.2 μm) of less than or equal to 20 ml/100 g, advantageously lessthan or equal to 15 ml/100 g or even less than or equal to 10 ml/100 g.As regards the V_(1 μm), it is beneficial for it not to exceed 7 ml/100g, or even 5.5 ml/100 g, preferably 4 ml/100 g. TheV_(0.1 μm)/V_(0.2 μm) ratio is preferably greater than or equal to 1.5,or even greater than or equal to 2, and even more advantageously greaterthan or equal to 2.5.

Under these conditions, and when the aluminas according to the inventionare in the form of beads having diameters of between 2.0 and 2.8 mm,they have a minimum S_(pp) of 2 daN, preferably equal to or greater than2.5 daN, or even equal to or greater than 3 daN, and even moreadvantageously exceeding 3.5 daN. As regards the shear test measuredunder the abovementioned conditions, this gives values of less than 4%,usefully less than 3% and even more advantageously less than 2% or evenless than 1%.

Table I gives the characteristics of several aluminas according to theinvention, obtained by varying the experimental conditions that havedetermined the forming of the beads and the following hydrothermaltreatment. Their particle size is in this case systematically between2.0 and 2.8 mm inclusive. A comparison is made with aluminas produced ina similar manner, with the same particle size, but without taking thesame care of controlling the porosity profile. All the specific surfaceareas were normalized, by calcination, to 105±5 m²/g in order to allow acomparison to be made that excludes the influence of this parameter.

Aluminas 1 to 8 are according to the invention, while aluminas 9 to 11are comparative examples not according to the invention.

The specific surface areas are given in m²/g, all the volumes are inml/100 g, the S_(pp) values are in daN and the measurements obtainedfrom the shear test are in % (cf. operating methods above). TABLE ICharacteristics of the aluminas described Specific surface Alumina areaV_(37 Å) V_(0.1 μm) V_(0.2 μm) V_(1 μm) V_(0.1 μm)/V_(0.2 μm) S_(pp)Shear I 1 110 93.9 19.4 6.6 2.7 2.9 3.4 0.8 n 2 104 92.2 30.3 9.4 3.23.2 3.0 0.5 v 3 105 84.3 14.0 6.9 2.1 2.0 4.5 0.7 e 4 103 87.2 27.8 13.53.8 2.1 3.3 1.5 n 5 106 87.2 26.4 17.5 5.3 1.5 3.0 1.6 t 6 105 101.230.5 15.5 5.0 2.0 2.9 1.8 i 7 110 86.2 7.0 2.7 2.1 2.6 6.6 0.5 o 8 10287.8 9.9 3.8 2.0 2.6 6.3 0.9 n Comparative 9 105 91.6 34.0 24.9 7.4 1.41.6 6.9 examples 10 100 87.2 31.3 21.6 7.1 1.4 2.0 4.3 11 104 85.7 32.322.5 5.9 1.4 1.8 4.9

As may be seen, precise control of the density of the pores of diametergreater than or equal to 0.1 μm and of the pores having diameters ofgreater than or equal to 0.2 4m is, unexpectedly and in a surprisingproportion, given the prior knowledge, essential for achievingagglomerates with the maximum mechanical strength independently of thelevel of total porosity of the alumina.

All the specimens according to the invention have a V_(37 Å) of greaterthan 75 ml/100 g and a V_(0.1 μm) of less than 31 ml/100 g. They alsohave a V_(0.2 μm) of less than 20 ml/100 g. They also have a V_(1 μm) ofless than 7 ml/100 g and a V_(0.1 μm)/V_(0.2 μm) ratio of at least 1.5,in accordance with the preferred characteristics of the invention.However, it should be pointed out that the features that have been givenas preferred are independent of one another and that it would remain inaccordance with the invention to comply with some of them withoutcomplying with the others.

It should be noted that the control specimens 9 to 11 all have a highV_(37 Å), in accordance with one of the obligatory characteristics ofthe aluminas of the invention. However, they have a V_(0.1 μm) and aV_(0.2 μm) that are higher than the maximum required by the invention,coupled with a V_(1 μm) of greater than the preferred upper limit of theinvention. This means that these specimens have a quantity oflarge-diameter pores that is relatively higher than that required by theinvention. However, these specimens have only relatively mediocremechanical properties as regards crush strength (S_(pp)) and resistanceto friction (shear).

This major influence of the presence of large-diameter pores is alsofound when the results of specimens 1 to 8 according to the inventionare examined. The best mechanical strength results are obtained forspecimens 7 and 8, the V_(37 Å) of which is not exceptionally high, andthese specimens are distinguished from the others by particularly lowV_(0.1 μm), V_(0.2 μm) and V_(1 μm) values. It has been found that thispore distribution gives these specimens a crush strength verysubstantially higher than that of the other specimens, although theirshear strength is at least very good. The best results are obtained withspecimen 7 that has the lowest V_(0.1 μm) and V_(0.2 μm) values of theseries.

It may also be noted that specimens (1, 2, 6) having a very highV_(37 Å), and therefore a very high overall porosity, but V_(0.1 μm),V_(0.2 μm), V_(1 μm) and V_(0.1 μm)/V_(0.2 μm) values within thenecessary or preferred limits of the invention, nevertheless havemechanical properties that are still substantially better than those ofthe control specimens. However, these have a lower overall porosity thatwould a priori lead to a better mechanical strength. Here again,controlling the presence of pores having diameters of greater than 0.1μm, or even 0.2 μm and 1 μm, under the specified conditions appears tobe particularly advantageous. Measuring the V_(0.1 μm)/V_(0.2 μm) ratiogives an indicator of what the distribution of the pore diameters in the0.1-0.2 μm range should be.

The invention is applicable for obtaining catalyst supports in the formof beads, especially those for the petroleum industry and thepetrochemical industry, in which the combination of a high porosity anda high mechanical strength of the supports gives effective catalystswith a long lifetime. These catalysts may be used, for example, inhydrotreatment, particularly in hydrometallation, to reduce the metalcontent of a hydrocarbon cut.

These alumina agglomerates can also be used as intrinsic catalysts or asadsorbents.

1. Alumina agglomerates of the type obtained by the treatment of analuminum hydroxide or oxyhydroxide, agglomeration of the alumina thusobtained, hydrothermal treatment of the agglomerates and calcination,wherein they have a V_(37 Å) of greater than or equal to 75 ml/100 g,preferably greater than or equal to 80 ml/100 g and even more preferablygreater than or equal to 85 ml/100 g; they have a V_(0.1 μm) of lessthan or equal to 31 ml/l00 g, preferably less than or equal to 25 ml/100g, even more preferably less than or equal to 20 ml/100 g and optimallyless than or equal to 15 ml/100 g; and wherein: they have a V_(0.2 μm)of less than or equal to 20 ml/100 g, preferably less than or equal to15 ml/100 g and even more preferably less than or equal to 10 ml/100 g.2. The alumina agglomerates as claimed in claim 1, wherein they have aV_(1 μm) of less than or equal to 7 ml/100 g, preferably less than orequal to 5.5 ml/100 g and even more preferably less than or equal to 4ml/100 g.
 3. The alumina agglomerates as claimed in claim 1 wherein theyhave a V_(0.1 μm)/V_(0.2 μm) ratio of greater than or equal to 1.5,preferably greater than or equal to 2 and even more preferably greaterthan or equal to 2.5.
 4. The alumina agglomerates as claimed in claim 1wherein they have simultaneously a V_(37 Å) of greater than or equal to80 ml/100 g, a V_(0.1 μm) of less than or equal to 15 ml/100 g, aV_(0.2 μm) of less than or equal to 10 ml/100 g, a V_(1 μm) of less thanor equal to 4 ml/100 g and a V_(0.1 μm)/V_(0.2 μm) ratio of greater thanor equal to 2.5.
 5. The alumina agglomerates as claimed in claim 1wherein they have been obtained from dehydrated hydrargillite.
 6. Thealumina agglomerates as claimed in claim 1 wherein they are in the formof beads.
 7. The alumina agglomerates as claimed in claim 1 wherein theyare in the form of extruded materials.
 8. The alumina agglomerates asclaimed in claim 1 wherein they are in the form of crushed materials. 9.The alumina agglomerates as claimed in claim 1 wherein they are in theform of monoliths.
 10. A catalyst support, especially for the petroleumof petrochemical industry, wherein it consists of alumina agglomeratesas claimed in claim
 1. 11. An intrinsic catalyst, especially for thepetroleum or petrochemical industry, wherein it consists of aluminaagglomerates as claimed in claim
 1. 12. An adsorbent, especially for thepetroleum or petrochemical industry, wherein it consists of aluminaagglomerates as claimed in claim
 1. 13. A process for producing aluminaagglomerates as claimed in claim 6, in which: an aluminum hydroxide oroxyhydroxide, preferably hydrargillite, undergoes flash dehydration inorder to obtain an active alumina powder; said active alumina powderundergoes a forming operation so as to obtain beads with a green filldensity of between 500 and 1100 kg/m³, preferably between 700 and 950kg/m³ inclusive, and a diameter predominantly between 0.8 and 10 mm,preferably between 1 and 5 mm; said beads undergo a heat treatment so asto provide them with a specific surface area of between 50 and 420 m²/g;said beads undergo a hydrothermal treatment by impregnation with wateror an aqueous solution, preferably aqueous acid solution, followed byresidence in an autoclave at a temperature of above 80° C.; and theagglomerates thus obtained are calcined, preferably between 500 and1300° C.
 14. The process as claimed in claim 13, wherein after the heattreatment that follows granulation, the alumina beads preferably have aV_(37 Å) of greater than 65 ml/100 g, preferably greater than 70 ml/100g.
 15. The process as claimed in claim 13 wherein the active aluminapowder obtained after dehydrating the aluminum hydroxide or oxyhydroxideis ground in order to obtain a powder with a median particle size d₅₀ ofpreferably between 5 and 25 μm.
 16. The process as claimed in claim 13wherein the active alumina powder obtained after dehydrating thealuminum hydroxide or oxyhydroxide is washed with water or an aqueousacid solution.
 17. The process as claimed in claim 13 wherein prior togranulation of said active alumina powder, the latter undergoes a flashoperation.
 18. The process for producing alumina agglomerates as claimedin claim 7, in which: an alumina-based material is mixed and extruded inorder to form it; the extruded materials thus obtained are subjected toa heat treatment so as to give them a specific surface area of between50 and 420 m²/g; said extruded materials are subjected to a hydrothermaltreatment by impregnation with water or with an aqueous solution,preferably an aqueous acid solution, followed by residence in anautoclave at a temperature of above 80° C.; and the agglomerates thusobtained are calcined, preferably between 500 and 1300° C.
 19. Theprocess as claimed in claim 18, wherein said alumina-based material isdehydrated hydrargillite.
 20. The process as claimed in claim 18,wherein alumina-based material comes from the precipitation of boehmite,pseudo-boehmite or bayerite, or a mixture of such materials.
 21. Theprocess as claimed in claim 13 wherein during the forming of saidalumina powder or of said alumina-based material, one or morepore-forming materials that disappear on heating are added to it. 22.The process as claimed in claim 21, wherein said pore-forming materialsare chosen from wood flour, charcoal, sulfur, tars, plastics oremulsions of plastics, polyvinyl alcohols, naphthalene.
 23. The processas claimed in claim 13 wherein said hydrothermal treatment is carriedout at a temperature of 150 to 270° C., preferably from 170 to 250° C.,for a time of greater than 45 minutes, preferably from 1 to 24 hours andeven more preferably from 1.5 to 12 hours.
 24. The process as claimed inclaim 13 wherein said hydrothermal treatment is carried out using anaqueous acid solution containing one or more mineral and/or organicacids.
 25. The process as claimed in claim 24, wherein said aqueous acidsolution also includes one or more compounds that can release anionscapable of combining with the aluminum ions.